[*] DB Support: Enable the postgresql driver with the following command:
[*] * This requires libpq-dev and a build environment
[*] $ gem install postgres
[*] $ gem install pg # is an alternative that may work

msf >

And you have tried installing mysql driver but didn't solve the problem ?

This happens because when you start msfconsole by /pentest/exploits/framework3/msfconsole, it does not set the environment variables properly

So all you have to do is, start msfconsole using BT5 menu or by using /usr/local/bin/msfconsole

Because Metasploit no longer supports databases other than the default PostgreSQL, there is no longer a need to set the driver. Thus db_driver is not useful and its functionality has been removed. Usually Metasploit will already have connected to the database; check db_status to see.